text: The last process in aerobic respiration is oxidative phosphorylation, also known as the electron transport chain. Here NADH and FADH deliver their electrons to oxygen and protons at the inner membranes of the mitochondrion, facilitating the production of ATP. Oxidative phosphorylation contributes the majority of the ATP produced, compared to glycolysis and the Krebs cycle. While the ATP count is glycolysis and the Krebs cycle is two ATP molecules, the electron transport chain contributes, at most, twenty-eight ATP molecules. A contributing factor is due to the energy potentials of NADH and FADH. A second contributing factor is that cristae, the inner membranes of mitochondria, increase the surface area and therefore the amount of proteins in the membrane that assist in the synthesis of ATP. Along the electron transport chain, there are separate compartments, each with their own concentration gradient of H + ions, which are the power source of ATP synthesis. To convert ADP to ATP, energy must be provided. That energy is provided by the H+ gradient. On one side of the membrane compartment, there is a high concentration of H+ ions compared to the other. The shuttling of H+ to one side of the membrane is driven by the exergonic flow of electrons throughout the membrane. These electrons are supplied by NADH and FADH as they transfer their potential energy. Once the H+ concentration gradient is established, a proton-motive force is established, which provides the energy to convert ADP to ATP. The H+ ions that were initially forced to one side of the mitochondrion membrane now naturally flow through a membrane protein called ATP synthase, a protein that converts ADP to ATP with the help of H+ ions.